the pulse cancels its own wake
the pulse cancels its own wake
Because a transmon qubit is only weakly anharmonic, the energy difference between state 0 and state 1 is dangerously close to the energy difference between state 1 and state 2. If a control system sends a fast, sharply-enveloped microwave pulse to flip the qubit from 0 to 1, the broad frequency bandwidth required to create that abrupt pulse will accidentally leak energy into the forbidden transition, exciting the qubit into state 2.
To drive the qubit quickly without leaking, quantum control electronics shape the signal using a DRAG (Derivative Removal by Adiabatic Gate) protocol. The system takes the mathematical derivative of the main pulse envelope, scales it, and applies it simultaneously to the out-of-phase (quadrature) microwave channel.
This derivative signal creates a precise destructive interference exactly at the 1→2 transition frequency, while leaving the desired 0→1 transition entirely unaffected. The speed of the pulse inherently creates a dangerous wake, but by calculating the derivative of that speed and transmitting it in parallel, the pulse is forced to emit a shadow that perfectly cancels its own wake.